EP3709726A1

EP3709726A1 - Method and system of wireless tdma communication for industrial machine-to-machine communication

Info

Publication number: EP3709726A1
Application number: EP19162695.1A
Authority: EP
Inventors: Pang Zhibo; Michele LUVISOTTO; Roger JANSSON
Original assignee: ABB Schweiz AG
Current assignee: Hitachi Energy Ltd
Priority date: 2019-03-14
Filing date: 2019-03-14
Publication date: 2020-09-16
Anticipated expiration: 2039-03-14
Also published as: EP3709726B1; CN111698666B; US20200295901A1; US11424890B2; CN111698666A

Abstract

Communication methods (100, 200, 300) for wireless TDMA communication using a super-frame structure (10A, 10B) between a first node (2) and at least one second node (3.k) is provided. In embodiments the second nodes (3.k) transmit payload data in uplink frames (ULk) that comprises a piggybacked field for time indicating data (T3.k), and preferably a field (ADLk) for acknowledging a received downlink transmission. The first node (2) is configured for calculating a clock offset and propagation delay based on the received time indicating data. In embodiments, the first node (2) is configured to transmit calculated clock offsets and propagation delay data, and preferably acknowledgements, in piggybacked fields of a downlink payload data frame, such as a beacon 10A, 10B, to the second nodes 3.1-3.n. These embodiments aims at providing high reliability and low latency for industrial machine-to-machine communications, where time indicating data, synchronization data, and acknowledgement are efficiently transferred and an efficient synchronization may provide absolute time useful for time critical control applications.

Description

Technical Field

The present invention relates to wireless communication, especially communication methods and systems for industrial monitoring and control purposes, such as monitoring and controlling an electric power system, monitoring and controlling equipment in an industrial manufacturing facility, and/or for performing other industrial automation control and monitoring purposes. The present invention further relates to TDMA (Time Division Multiple Access) based wireless communication using a frame structure for scheduling of time slots, and especially to synchronization.

Background and Prior Art

An example of industrial communication related to the present invention is communications for the monitoring and control of electric power grids, wherein wired communications, compliant with IEC 61850, may be used that provide time slots of some micro-seconds or lower. In, for example, electric substation automation for the control and monitoring of electric power distribution, the requirements for reliability and low latency are comparably high.
However, using wired communications has drawbacks in cost. Wireless alternatives could be cheaper, e.g. in installation, and are sought after for in industrial automation. When using a TDMA scheduled wireless communication, it is important that the equipment is synchronized in order to avoid communication collisions. Also, wireless communication risks being subjected to disturbances and reliably acknowledging correct reception of data can also be considered of high importance in industrial automation and similar systems.
US2011/0211461 (E1 ) describes a wireless communication network aiming at keeping latency low at the same time as effectively handling synchronization and acknowledgement. E1 discuss networks where sensor nodes transfer data to a central base station. E1 describes a frame structure for the wireless communication, wherein synchronization messages are included in a downlink beacon frame. Especially, E1 is related to retransmissions of lost data. A previous publication that E1 refers to; Bahr M. et al Proposal for Factory automation, September 2008 (E2), describes using group acknowledgment incorporated in a downlink beacon frame from the base station to the nodes. Thus, a number of acknowledgements are arranged within the beacon frame, see for example §0005, §0008-§0009, §0012 of E1, and said downlink (DL) beacon frame includes a synchronization message and may also include a sequence of acknowledgements to a group of nodes, see for example §0037, last sentence, and §0038, in E1.
Both E1 and E2 concerns TDMA communication where downlink and uplink data messages are scheduled according to a super-frame structure that is periodically repeated over time. Using group acknowledgements included in a downlink beacon frame is a way to effectively handle acknowledgements of uplink transmissions. Also, including a synchronization message in the downlink beacon contributes to efficient scheduling. However, little information is given in E1 about the uplink transmissions. E1 seems mainly targeting uplink data collection from sensor nodes to a base station or gateway. Few details are given on the synchronization, see for example §0011 and §0033 - §0034 of E1. The synchronization seemingly consists of the start of a super-frame by means of a downlink transmission, possible the downlink beacon includes an indication of the start of a new super-frame. E1 does not present any specific measures for handling propagation delay.
Thus, there is a need for alternative and/or additional measures for developing the scheduling of communication that especially includes handling the uplink transmissions. Measures suitable for contributing to reliability and avoiding latency would be appropriate.

Summary of Invention

An aim of the present invention is to provide a method for wireless communication suitable for industrial appliances, such as industrial automation, which wireless communication can contribute further to efficient scheduling of the wireless communication.
For this purpose, a first aspect of the present invention provides a communication method for wireless communication using a TDMA super-frame structure for scheduling uplink and downlink transmissions between a first and a second communication node. The communication method is performed by the communication node, which second node comprises a clock. The communication method includes receiving a downlink transmission and transmitting an uplink response,. The transmitting of uplink response is performed in an uplink frame of the super-frame structure, which uplink frame comprises a field configured for uplink payload data. Especially, the uplink frame comprises a field for time indicating data, wherein the transmitting of the uplink response includes transmitting time stamps in the time indicating data field, which time stamps indicates the time instance of the receiving of the downlink transmission, and the time instance of the transmitting of the uplink response, which time instances is determined by means of said clock of the communication node.
Using fields in the uplink frames for such time indicating is an efficient way to transfer communication relevant data to the uplink, or first, node. Also, by letting the time stamps indicate reception of the downlink and transmission of the uplink in accordance with the clock of the node provides information about how well the clock is synchronized and information about the transmission time for the communication.
In an embodiment of the first aspect, the uplink frame comprises a field for acknowledgements and the transmitting of the uplink response includes transmitting an acknowledgement indicating the reception of the downlink transmission.
Including an acknowledgement in a field of the uplink frame contributes further to efficient communication; said acknowledgements contributing to reliability, which when included as a field in the frame has a limited effect on latency. For example, and preferably, each acknowledgement can be indicated by a single bit.
In an embodiment of the first aspect, the received downlink transmission comprises synchronization data and the method further comprises synchronizing the clock based on the synchronization data, wherein the synchronizing is performed between the receiving of the downlink transmission, and the transmitting of the uplink response, and the method includes adjusting said time instances in accordance with the synchronization data.
An alternative or additional embodiment of the first aspect comprises receiving a further downlink transmission, which further downlink transmission comprises synchronization data, wherein the method includes synchronizing the clock based on the synchronization data of the further downlink transmission.
In a second aspect, the present invention provides a method for wireless communication using a TDMA super-frame structure for scheduling uplink and downlink transmissions between a first, or uplink, communication node and a second, or downlink, communication node. The communication method is performed by the first communication node, which first node comprises a clock. The communication method includes transmitting a downlink beacon to at least one second node and receiving an uplink response from the at least one second node of the second communication nodes. Especially, the communication method includes:

registering the time instance of transmitting the downlink beacon,
registering the time instance of receiving the response,

calculating a clock offset between the first communication node and the second node,
calculating the propagation delay between the first communication node and the second node, which calculating of clock offset and calculating of propagation delay is based on the registered time instances of the downlink transmission and uplink response and on the time stamps indicating time of reception of the downlink beacon and indicating the time of transmission of the response, and
transmitting a further downlink beacon and including indicating said calculated clock offset and preferably said calculated propagation delay in a field for synchronization data provided in the downlink beacon.

In a third aspect, the present invention provides a communication node comprising a communication interface for communicating with another communication node, a clock and registering means for registering times of reception and transmission and time stamping transmissions. The communication node of the third aspect is configured to perform the communication method of the first aspect.
Preferably the communication node of the third aspect further comprises a synchronizer for adjusting the clock, and is configured to perform the synchronizing of the method of the first aspect.
In a fourth aspect, the present invention provides a communication node comprising a communication interface for wireless communication with other communication nodes, a clock for indicating time, a time register for registering times of receiving and transmitting, a time offset calculator for calculating a time offset of a clock and a propagation delay calculator for calculating a propagation delay between communication nodes, which communication node is configured to perform the communication method of the second aspect.
In an embodiment of the fourth aspect, the communication node further comprises acknowledging means for acknowledging received transmissions, wherein the communication node is configured to include acknowledgements of received uplink frames in a field in the downlink beacons transmitted.
According to a fifth aspect, the present invention provides a communication method for wireless communication, which communication method uses TDMA (Time Division Multiple Access) for communication between a first node and a plurality of second nodes. The TDMA communication uses a super-frame structure for scheduling uplink data transmissions from the second nodes to the first node and scheduling downlink data transmissions from the first node to the second nodes. The communication method comprises:

transferring a downlink beacon from the first node to the second nodes,
transferring an uplink transmission as a response to the downlink transmission from one second node, of the second nodes, to the first node.

The downlink beacon preferably consisting of a first frame in accordance with the super-frame structure, and said uplink transmission preferably comprising a second frame in accordance with the super-frame structure.
Especially, the communication method, of the fifth aspect, comprises:

calculating a clock offset and a propagation delay between the first node and the second node using the time instance of transmitting the downlink beacon by the first node, the time instance of receiving the downlink beacon by the second node, the time instance of transmitting the response by the second node and the time instance of receiving the response in the first node (2), and
synchronizing the first node and the second node, which synchronization includes transferring a subsequent downlink beacon to the second node including an indication of said clock offset and said propagation delay in a synchronization field of the downlink beacon.

In an embodiment of the fifth aspect, the present invention further includes:

registering the time instance of transfer of the first frame by the first node as the time instance of transmitting and by the second node as the time of reception, and
registering the time of transfer of the response frame, as the time of transmitting by the second node and the time of receiving by the second node, wherein said calculating of offset and delay being based on said registered time instances.

In an embodiment of the fifth aspect, the communication method further comprises including, in the response, a first and a second time stamp in a time indicating field of the second frame, which first time stamp indicates the time of reception of the first beacon, and which second time stamp indicates the time of transmission of the response.
In an embodiment of the fifth aspect, the communication method further comprises including, in the response, an acknowledgement of the reception of the downlink beacon in an acknowledgement field of the second frame.
According to a sixth aspect, the present invention provides a communication system for wireless communication using TDMA (Time Division Multiple Access) between a first node and a plurality of second nodes. The communication system comprises a super-frame structure for scheduling uplink data transmissions from the second nodes to the first node and scheduling downlink data transmissions from the first node to the second nodes. The super-frame structure comprises a first frame configured for a downlink beacon and a set of second frames, each second frame dedicated for uplink transmissions from a respective one of the plurality of second nodes to the first node. Especially, each second frame comprises a field configured for transmitting time indicating data to the first node, or a fraction of the second frames comprises a field configured for transmitting time indicating data to the first node.
In an embodiment of the sixth aspect, the time indicating data comprises a first and a second time stamp, where the first time stamp indicates reception of a downlink frame, e.g. the beacon, from the first node and the second time stamp indicates transmission of an uplink frame, e.g. the respective second frame of each second node, to the first node.
Preferably, and for each respective second node, in an embodiment of the sixth aspect, each second frame comprises a field configured for acknowledgement of reception of downlink data from the first node. Thus, said super-frame includes a field for acknowledgement in the uplink second frame for acknowledging the downlink first frame, e.g. the beacon.
In a further embodiment of the embodiment of the sixth aspect where each second frame comprises a field configured for transmitting time indicating data to the first node, the first frame comprises a field configured for transmitting synchronization data to each one of the second nodes.
In a further embodiment of the alternative embodiment of the sixth aspect where a fraction of the second frames comprises a field configured for transmitting time indicating data to the first node, the first frame comprises a field configured for transmitting synchronization data to the corresponding fraction of the second node, so that for each time indicating field received in the fraction of the second frames, the first frame, preferably the beacon, is configured for transmitting synchronization data.
In a further embodiment of these synchronization data embodiments of the sixth aspect, the synchronization data of the synchronization field is configured for indicating a propagation delay of transmissions between the first node and the second node and a clock offset of the clock of the second node compared to the first node.
In an embodiment of the sixth aspect, the first node and each second node comprises a respective clock, and said first node being configured to calculate the clock offset of a second node in accordance with the following equation: $Δ = ({T^{3}}_{0} - {T^{2}}_{0} + {T^{3}}_{1} - {T^{2}}_{1}) / 2$
where

Δ is the clock offset of the second node in question;
T² ₀ is the time instance when the first node transmits a downlink frame, such as the beacon the time instance being determined by the first node (2);
T² ₁ is the time instance when the first node receives the uplink frame from the second node according to the clock of the first node;
T³ ₀ is the time instance when the second node receives the downlink frame, e.g. the beacon, according to the clock of the second node and as provided to the first node by means of the first time stamp;
T³ ₁ is the time instance when the second node transmits the uplink frame according to the clock of the second node, and as provided to the first node by means of the second time stamp.

In an embodiment of the sixth aspect the first node is configured to calculate the propagation delay d in accordance with the following equation: $d = {T^{3}}_{0} - {T^{2}}_{0} - Δ$
where
d is the propagation delay between the first node and the second node (3.k) in question, and wherein $Δ = ({T^{3}}_{0} - {T^{2}}_{0} + {T^{3}}_{1} - {T^{2}}_{1}) / 2.$
In an embodiment of the sixth aspect, the first frame, for each second node, comprises a field configured for acknowledgement of the reception of a second frame of a previous super-frame, in order to acknowledge the uplink second frame of a previous super-frame.
The communication system, preferably the first node, is configured for determining the propagation delay and the clock offset based on the time indicating data of each second node, especially the time stamps received from the second node. The propagation delay for each second node is transmitted to each respective second node, preferably from the first node, and the second node is configured to use the propagation delay for timing future transmissions.
The super-frame may include a set of third frames, where each third frame is dedicated for downlink data to a respective node of the plurality of nodes.

Brief Description of Drawings

Figure 1 illustrates a wireless communication network;
Figure 2A illustrates an embodiment of a super-frame structure;
Figure 2B illustrates an embodiment of a super-frame structure;
Figure 3A illustrates an embodiment of the downlink beacon of the super-frame corresponding to the embodiment of figure 2A;
Figure 3B illustrates an embodiment of the downlink beacon of the super-frame corresponding to the embodiment of figure 2B;
Figure 3C illustrates further embodiments of the downlink embodiments of figure 3A and 3B;
Figure 4A illustrates a set of uplink frames according to an embodiment of the present invention related to the super-frame structures of the embodiments of figures 2A and 2B;
Figure 4B illustrates an alternative to the embodiment of figure 4A;
Figure 4C illustrates an alternative to the embodiment of figure 4A, which embodiment comprises uplink frames that correspond to the downlink beacon in the embodiment of figure 3C;
Figure 5 illustrates transmissions between a first node and a second node;
Figure 6 illustrates a method of communication according to an embodiment of the present invention;
Figure 7 illustrates an embodiment of methods of communication according to the present invention, as performed by means of a master node and a slave node.
Figure 8 illustrates an embodiment of a master node, or first node configured for performing part of the method of figure 7;
Figure 9 illustrates an embodiment of a slave node, or second node configured for performing a corresponding part of the method of figure 7.

Description of Embodiments

Figure 1 illustrates a wireless communication network 1 having a star configuration where a first node 2 acts as a central control node for number of second nodes 3. Such a first node 2 may also be referred to as a master node, and the second nodes 3 as slave nodes. Figure 1 illustrates uplink and downlink communication between the first node 2, or master node, and the second nodes 3, or slave nodes. The second nodes 3 are numbered 3.1 to 3.n, and are ordered during communication but are essentially similar to each other.
Figure 2A illustrates a super-frame structure 10A employed according to embodiments of the present invention. In the top view in figure 2A, the super-frame 10A comprises a first frame in the form of a beacon 11A for downlink transmission to the nodes 3. The super-frame 10A further comprises a number of downlink frames DL1-DLn dedicated for downlink transmissions to respective ones 3.1; 3.2; 3.3; ...; 3.n of the nodes 3.1 - 3.n. The dedicated downlink frames DL1-DLn are followed by a set of dedicated uplink frames UL1-ULn configured for transmissions from each respective second node 3.1; 3.2; 3.3; ...; 3.n to the first node 2. The super-frame 10A also includes a shared frame 13, preferably for uplink transmissions.
The beacon 11A, the dedicated downlink frames DL1-DLn, and the dedicated uplink frames UL1-ULn are shown in more detail in the lower view of the super-frame 10A in figure 2A, where the shared frame 13 is omitted. Especially, the lower super-frame 10A in figure 2A illustrates that the beacon 11A and each dedicated uplink frame UL1, ..., ULn includes specific fields. These fields designate fields used in an embodiment of the present invention and these fields are further illustrated in figures 3A and 4.
Each dedicated downlink frame DL1, DL2, ..., DLn, in the embodiment of figure 2A, comprises a respective field DD1, DD2, ..., DDn configured for downlink data, which can be referred to as payload data or operational data. Operational data and payload data herein refer to data for monitoring or control of the process, such as the industrial process, being monitored and/or controlled. The operational/payload data being in contrast to acknowledgments and synchronization data used for controlling the communication.
Figure 2B illustrates a super-frame structure 10B employed according to alternative embodiments of the present invention. In the top view in figure 2B, the super-frame 10B comprises a first frame in the form of a beacon 11B for downlink transmissions to the nodes 3. The beacon frame 11B is followed by a set of dedicated uplink frames UL1-ULn configured for transmissions from each respective second node 3.1, 3.2, 3.3, ..., 3.4 to the first node 2. The super-frame 10B also includes a shared frame 13, preferably for uplink transmissions.
The downlink beacon 11B of the embodiment of figure 2B comprises a number of downlink fields DD1-DDn, as illustrated in the lower view of figure 2B, each downlink field DD1, DD2, ..., DDn dedicated for downlink payload transmissions to respective ones of the nodes 3.1, 3.2, 3.3, ..., 3.n.
Figures 3A, 3B illustrate the downlink beacons 11A, 11B, respectively, in greater detail. The downlink beacons 11A, 11B, of both figures 3A, 3B, include a field AUL1-AULn for acknowledgements of previous respective uplink transmissions, i.e. acknowledgement of each uplink frame UL1, UL2, ... , ULn, and a field S3.1-S3.n for respective synchronization data to individual ones of the second nodes 3.1, 3.2, ..., 3.n.
The field for acknowledgements AUL1-AULn is provided for the first node 2 to acknowledge the reception of data in the uplink frames UL1-ULn, one acknowledgement for each of the second nodes 3.1, 3.2, 3.3, ..., 3.n. Each acknowledgment preferably has the size of one bit and, .i.e., the total size of the field AUL1-AULn for acknowledgements is n bits for acknowledging reception from the n second nodes 3.1-3.n.
The number of synchronization fields may preferably be as high as the number of second nodes 3.1-3.n to enable accurate synchronization. To provide fast and reliable communication it is important to synchronize often, and in many industrial applications synchronization should be performed as often as every super-frame 10A, 10B and contribute to the control the industrial process with high accuracy.
However, in another embodiment where the applications in question require less strict need for synchronization, the number of synchronization fields configured can be lower than the number of second nodes 3.1-3.n. The communication interface can be more efficiently used if the number of synchronization fields are limited, and each second node 3.1, 3.2, 3.3, ..., 3.n may in some applications not need synchronization for each super-frame 10A, 10B. For example, the number of synchronization fields can be selected as half (n/2) of the number (n) of second nodes 3.1-3.n, so that each second node 3.k is synchronized every second super-frame 10A, 10B. Figure 3C illustrates the embodiment where the number m of synchronization fields S3.1-S3.m are lower than the number n of second nodes 3.1-3.n.
The field for synchronization data S3.1-S3.n or S3.1-S3.m comprises data slots for transmitting synchronization data from the first node to each second node 3.1, 3.2, 3.3, ..., 3.n. The synchronization data for each second node 3.1, 3.2, 3.3, ..., 3.n is determined by the first node on the basis of time indicating data received from the second node 3.1, 3.2, 3.3, ..., 3.n in question. The determining of the synchronization data for transmission in the synchronization field S3.1-S3.n or S3.1-S3.m will be described further in relation to figure 5.
The embodiment of figure 3B differs from the embodiment of figure 3A in that the beacon 11B comprises downlink payload data fields DD1-DDn, each dedicated for a specific second node 3.k. In the embodiment of figure 3A said payload data fields DD1-DDn are provided in the dedicated downlink frames DL1-DLn. In the embodiment of figure 3C, the payload data fields DD1-DLn are optional, to illustrate that the embodiment of figure 3C can be provided in any of the embodiments of figures 3A and 3B.
Figures 4A-C illustrate how acknowledgements and time indicating data are piggy-backed to the uplink payload transmissions.
Each uplink frame UL1, UL2, ..., ULn of the set of uplink frames UL1-ULn of figure 4A-C includes a field UD1, UD2, ..., UDn configured for uplink operational, or payload, data. Each uplink frame UL1, UL2, ..., ULn of the set of uplink frames UL1-ULn of the embodiment of figure 4A includes two specific fields; one field ADL1, ADL2, ..., ADLn for acknowledgement, and one field T3.1, T3.2, ..., T3.n for time indicating data. Each uplink frame UL1, UL2, ..., ULn of the set of uplink frames UL1-ULn of the embodiments of figures 4B-C includes one specific field; one field ADL1, ADL2, ..., ADLn for acknowledgement. A fraction, such as 1/2, 1/3, ..., 1/n, of the uplink frames UL1-ULn in the embodiment of figure 4C includes a field T3.1, T3.2, ..., T3.m for time indicating data. The embodiment of figure 4B illustrates m = n/2; i.e. half of the uplink frames UL1-ULn comprises a field T3.1, T3.2, ..., T3.(n/2) for time indicating data.
In these embodiments of figures 4B-C, the number of time indicating fields T3.1-T3.(n/2) and T3.1-T3.m is limited to less than every uplink frame UL1-ULn, these time indicating fields of the super-frame being configured for applications where synchronization is performed in every super-frame, but not for every second node 3.1 - 3.n. The number of time indicating fields in these uplink frames correspond to the number of synchronization fields in the downlink frames, see example of figure 3C. The embodiment of figure 4B illustrate the special embodiment wherein each second node 3.1, 3.2, 3.3, ..., 3.n is synchronized in every second super-frame 10A, 10B and transmit time indicating data in every second super-frame 10A, 10B.
Each second node 3.1, 3.2, 3.3, ..., 3.n should be configured to use its acknowledgement field ADL1, ADL2, ..., ADLn to acknowledge the reception of the beacon 11A, 11B and/or the dedicated downlink frame DL1, DL2, ..., DLn, including acknowledging reception of the payload data in the dedicated downlink field DD1; DD2; ...; DDn. Each second node 3.1, 3.2, 3.3, ..., 3.n should be configured to use the field for time indicating data T3.1, T3.2, ..., T3.n, or T3.1, T3.2, ..., T3.m, to transfer data indicating the time of its reception of downlink data, e.g. downlink beacon 11A, 11B or payload data of respective field DD1; DD2; ...; DDn, and the time of its transmission of uplink data UL1; UL2; ...; ULn, e.g. the uplink payload data field UD1; UD2; ...; UDn. The received time indicating data will be used by the first node 2 to provide synchronization data to each second node 3.1; 3.2; 3.3; ...; 3.n. Suitably, each second node 3.1; 3.2; 3.3; ...; 3.n is configured to use the time indicating data field T3.1; T3.2; ...; T3.n to time stamp reception of the beacon 11A, 11B and the transmission time instant of the uplink frame UL1;,UL2, ..., or ULn. These time stamps should subsequently be used by the first node 2 to provide synchronization data, such as propagation delay and clock offset data, to each second node 3.1, 3.2, 3.3, ..., 3.n in the synchronization field S3.1-S3.n, or S3.1-S3.n, of a following beacon 11A, 11B. In the embodiment of figure 3C, the synchronization data are provided in the synchronization fields S3.1 - S3.m to each node 3.1 - 3.m that is scheduled for synchronization in the present super-frame.
Preferably, each synchronization field S3.1, S3.2, ..., S3.n (or S3.1, S3.2, ..., S3.m) from the first node 2 comprises an indication of a respective propagation delay and an indication of a respective time offset of respective ones of the second node 3.1, 3.2, ..., 3.n, which propagation delay and time offset is derived using the time indicating data received from respective ones of the second node 3.1, 3.2, 3.3, ..., 3.n.
A preferred embodiment for the calculation of a propagation delay and a time offset is given in equations 1 and 2, and with reference to figure 5. In figure 5, the master node, or first node 2, transmits a beacon frame 11A, 11B at time instance T² ₀ and receives an uplink frame ULk at time instance T² ₁. The superscript "2" in T² ₀ and T² ₁ indicates the time according to a clock of the first node 2. The beacon frame 11A, 11B is received by each second node 3.1; 3.2; ...; 3.n, as exemplified by the slave node, or second node 3.k, in figure 5. The second node 3.k receives the beacon frame 11A, 11B at time instance T³ ₀, and transmits an uplink frame ULk as a response at time instance T³ ₁. The superscript "3" in T³ ₀ and T³ ₁ indicates that the time is provided by the second node 3.k. The second node 3.k is configured to time stamp the time instances T³ ₀ and T³ ₁ and include them in the uplink frame ULk in the previously described time indicating data field T3.k
A time offset Δ, for a respective second node 3.1, 3.2, 3.3, ..., 3.n, can be calculated as follows: $Δ = ({T^{3}}_{0} - {T^{2}}_{0} + {T^{3}}_{1} - {T^{2}}_{1}) / 2$
where

T² ₀ is the time instance when the first node 2 transmits the beacon 11, and where the time instance is determined according to the first node 2;
T² ₁ is the time instance when the first node 2 receives the uplink frame ULk from the second node 3.k (where "k" denotes any of 1 to n) according to the time of the first node 2;
T³ ₀ is the time instance when the second node 3.k receives the beacon 11 according to the time of the second node 3.k;
T³ ₁ is the time instance when the second node 3.k transmits the uplink frame ULk according to the time of the second node 3.k

The propagation delay "d" for the respective second node 3.1, 3.2, 3.3, ..., 3.n, can be calculated as follows: $d = {T^{3}}_{0} - {T^{2}}_{0} - Δ$
with Δ according to equation 1.
The second node 2 is configured to calculate the time offset Δ and propagation delay d for the second node 3.k and include the time offset Δ and the propagation delay d in the synchronization field S3.1-S3.m of the following beacon 11A, 11B, denoted "Next DL" in figure 5.
The two-way synchronization of the present invention provides an absolute time in the communication system, and facilitates synchronized control of an underlying industrial process, such as time-critical industrial automation processes.
The downlink and uplink transmissions of figure 5 are described in further detailed embodiments in figure 6 and figure 7, including a specific emphasis on synchronization. Figure 7 illustrates a communication method 100 wherein the communication method 200 to the left in the figure is performed by the first node 2, and the communication method 300 to the right is performed by any of the second nodes 3.1 - 3.n, i.e. a slave node, or second node 3.k.
The communication method 100 of figure 6 includes transferring 102 a downlink transmission, including a first frame (11A, 11B or DLk), preferably a beacon 10A, 10B, from a first node 2 to a second node 3.k. The communication method comprises registering 103 the time instant of transmitting T² ₀ and the time instant of receiving T³ ₀ the first frame, such as the beacon 11A, 11B. The registering 103 of the time instant of transmitting T² ₀ the first frame being made according to a "clock" of the first node 2. The registering 103 of the time instant of receiving the first frame 11A, 11B T³ ₀ being made according to a "clock" of the second node 3.k. The TDMA schedule used in the node 2, 3.k can be seen as such a "clock", and the consequence of the synchronization is that the TDMA schedule of the node, preferably the slave node, being adjusted. The communication method 100 preferably includes registering 105 the time instant T³ ₁ of a response frame transfer as a time stamp in the response frame ULk, which registering is performed by the second node 3.k.
The first frame, preferably the beacon frame 11A, 11B, includes a synchronization data field S3.k, an acknowledgement field AULk for acknowledging reception of a previous uplink transmission, and a payload data field DDk, as previously described with reference to the super-frame structure 10A, 10B in figures 1 to 3B.
The response comprises transferring 106 a second frame, e.g. the uplink frame ULk, from the second node 3.k to the first node 2. Transferring 106 of the second frame ULk is preferably time stamped and the transfer includes the time stamp of the transfer of the first frame, according to the clock, or time, of the second node 3.k, and also includes the time stamp of the transfer of the second frame according to the clock of the second node 3.k. The method further includes registering 107 the time T² ₁ T³ ₁ the second frame ULk is transferred. The registering 107 of the time instant of transferring T² ₁ T³ ₁ the second frame ULk being made according to the clock of the first node 2 and the time of the second node 3.k. The time of transmission T³ ₁ according to the clock of the second node 3.k, and the time of reception T² ₁ according to the clock of the first node 2. Thus, preferably the first node 2 obtains the time instances of both transferred frames 11A, 11B, and ULk as registered by both nodes 2, 3.k in relation to their respective clock; i.e. T² ₀ T³ ₀ T² ₁ and T³ ₁. These registered time instances are subsequently used for synchronization of the first 2 and the second node 3.k. The clock of the second node 3.k, which is a slave node, being adjusted in order to correspond to the time of the clock of the first node 2, which is the master node. Adjusting clock of every second node 3.1; 3.2; ...; 3.n provides a absolute time for the communication system; i.e. the first node 2 and all second nodes 3.1-3-n will have the same time. In time-critical operations performed in many industrial applications, an absolute time provided by the machine-to-machine communication enables controlling, e.g. synchronizing, actions taken in the industrial operations controlled.
The transferring 106 of the response to the first frame 11A, 11B, DLk is performed by means of the transfer of the second frame ULk, which comprises an acknowledgement field ADLk for acknowledging reception of the first frame 11A, 11B, DLk, and a payload data field UDk for transfer of operational data to the first node 2, in addition to the time indicating fields T3.k that includes said time stamps.
The communication method 100 further includes calculating 108 a time offset, such as the deviation of the clock, e.g. TDMA schedule, of the second node 3.k compared to the clock, e.g. TDMA schedule, of the first node 2. The calculation 108 of time offset is preferably made in accordance with equation 1.
The communication method 100 further includes calculating 110 the propagation delay between the first node 2 and the second node 3.k, and preferably the propagation delay is determined in accordance with equation 2.
The communication method preferably also includes synchronizing the clocks, preferably setting the clock of at least one second node 3.1-3.n, which means that the TDMA schedule of the at least one second node 3.1-3.n is adjusted. The synchronization 104 being performed by means of synchronization data added to a synchronization field S3.1-S3.m of the first frame 11A, 11B or DLk. The synchronization data being calculated from a previous transfer. Each clock of the second nodes 3.1-3.n may preferably be adjusted each super-frame 10. In other preferred embodiments, each clock is set for every mth super-frame 10, e.g. every second, every third, every fourth, ..., or every nth super-frame.
The communication method 100 further includes transfer 112 of a further frame, preferably a following beacon 11A, 11B, from the first node 2 to the second node 3.k. This transfer 112 is a repetition of the transfer 102 of the first frame 11A, 11B, DL, includes synchronization data in fields S3.1-S3.m of the previous downlink (e.g. 11A, 11B) and uplink (e.g. ULk) frames, preferably as calculated in accordance with equations 1 and 2.
The communication method 100 continues with synchronization 114 of the two nodes, preferably the clock, or TDMA schedule, of second node 3.k is adjusted 104/114 based on the calculated (108) time offset and the calculated (110) propagation delay.
Figure 7 illustrate a preferred embodiment, and especially the measures (communication method 200) performed by means of the master node, or first node 2; and the measures (communication method 300) performed by the slave node, or second node 3.k. The communications methods 200, 300 correspond to the overall communication method 100, and the correspondence is indicated by the reference numerals of the communication method 100 to the left in figure 7. For example, the transferring 102 of the first frame 11A, 11B, DLk corresponds to the transmitting 202 of a first downlink frame, or beacon 11A, 11B from the first node 2 and to the receiving 302 of the first downlink frame, or beacon 11A, 11B, by the second node 3.k.
The communication method 200 performed by the first node 2, or master node, comprises transmitting 202 a downlink frame, e.g. the beacon 11A, 11B, at time instance T² ₀ and registering 203 the time of transmission. The communication method 200 performed by the first node 2 further comprises receiving 206 an uplink response at time T² ₁. The uplink response ULk includes time stamps indicating reception of the first frame 11A, 11B, DLk, i.e. T³ ₀, and transmission of the response ULk, i.e. T³ ₁. The communication method continues with calculating 208 the time offset and calculating 210 the propagation delay based on the received time stamps and its own registered times of transmission T² ₀ and reception T² _1. The calculated time offset and propagation delay is indicated in the next DL transmission 212. Each downlink transmission 202, 212, illustrated by the downlink transmission 202 and the next downlink transmission 212, preferably includes synchronization data to at least one of the second nodes 3.1; 3.2, ..., 3.n calculated from the previous downlink and uplink transfers. Each downlink transmission 202, 212 also includes acknowledgement fields AULk for acknowledging the received previous uplink transmission, and a payload data field DDk.
The method 300 performed by the second node 3.k comprises synchronizing 304 / 314 of its TDMA schedule based on synchronization data received in the downlink transmission 11a, 11B, DLk. Thus, the method 300 performed by the second node 3.k comprises receiving 302 a downlink transmission including synchronization data in a field S3.1, S3.2, ..., S3.m, which synchronization data is used by the second node 3.k to synchronize its clock, i.e. adjust its clock in accordance with the time of the first node 2. The received 302 downlink transmission, e.g. beacon 11A, 11B, also includes the described acknowledgements of acknowledgement field AULk and downlink payload data in field DDk.
The method 300 performed by the second node 3.k further comprises an uplink transmission 306 of payload data, in field UDk, that may include time stamps in a time indicating field T3.1-T3.m corresponding to the time T³ ₀ of reception of the downlink transmission and the time T³ ₁ of transmission of the uplink response, and acknowledgement of the received downlink transmission in acknowledging fields ADLk. The uplink transmission 306 should include time stamps for each second node 3.1-3.n that should be synchronized in the following super-frame 10A, 10B and obtain synchronization data when receiving 312 the following downlink transmission, e.g. when receiving 312 the next beacon 11A, 11B.
Figures 8 and 9 illustrates communication nodes configured to perform the described methods. Figure 8 disclose a master node, or second node 2. Figure 9 disclose a slave node, or second node 3.k. Each communication node of figures 8-9 are configured to perform its parts of the methods illustrated in figures 5-7. The first node 2 of figure 8 comprises a wireless communication interface 21 for communication with the second nodes 3.1 - 3.n. The second node 3.k of figure 9 comprises a wireless communication interface 31 for communication with the first node 2. The first node 2 and second node 3.k constitute a combination of hardware and software, and are illustrated as comprising means for performing the above previously disclosed methods. The first node 2 and second node 3.k may also include an interface to an underlying industrial process, for example the first node is communicatively connected to a wired communication system of the industrial process, and the second node 3.k operationally connected, by an interface, to sensor means and/or to industrial process equipment, however such interfaces are not explicitly illustrated.
The first node 2 of figure 8 further comprises a clock 21 and a time register unit, or time register 23. The first node 2 comprises the clock 21 and the time register 23 in order to perform the registering of transmission times and reception times, and is thereby configured to perform the registering of times of steps 203, 206 used when performing the calculation of offsets (208) and calculation of propagation delay (210) for each second node in the communication system. The first node also includes calculation means for these calculations, including a time offset calculator 24 and a propagation delay calculator 25. In order to provide efficient and reliable communication, the first node further includes acknowledge means 27 to acknowledge uplink transmissions and include these acknowledgements, preferably one bit per second unit 3.k, in a field of a downlink frame, such as the beacon 11A, 11B.
The second node 3.k of figure 9 further comprises a clock 31 provided for time indication, scheduling its communication, and synchronization to the clock 21 of the first node 2, and thereby to the communication system 1. The second node 3.k further comprises a synchronizer 36 for performing said synchronization. The second node 3.k further comprises a register unit, or registering means 33 in order to register times of reception and transmission, configured for the registering 303, 103, and time stamping 305, 105 performed by the second nodes and illustrated in of figures 6 and 7.
Embodiments of communication methods 100, 200, 300 for wireless TDMA communication using a super-frame structure 10A, 10B between a first node (2) and at least one second node 3.k has been provided. In embodiments the second nodes 3.k transmit payload data in uplink frames ULk that comprises a piggybacked field for time indicating data T3.k, and preferably a field ADLk for acknowledging a received downlink transmission. The first node 2 is configured for calculating a clock offset and propagation delay based on the received time indicating data. In embodiments, the first node 2 is configured to transmit calculated clock offsets and propagation delay data, and preferably acknowledgements, in piggybacked fields of a downlink payload data frame, such as a beacon 10A, 10B, to the second nodes 3.1-3.n. These embodiments aims at providing high reliability and low latency for industrial machine-to-machine communications, where time indicating data, synchronization data, and acknowledgement are efficiently transferred and an efficient two-way synchronization process may provide absolute time useful for time critical control applications. The present invention is, however, not limited to these embodiments, but may be varied by the skilled person within the scope of the claims.

Claims

A communication method (300) for wireless communication using a TDMA (Time Division Multiple Access) super-frame structure (10A, 10B) for scheduling uplink and downlink transmissions between a first node (2) and a second node (3.k) that comprises a clock (31), which communication method (300) is performed by the second node (3.k), wherein the communication method (300) includes receiving (302) a downlink transmission and transmitting (306) an uplink response, said transmitting (306) of uplink response is performed in an uplink frame (ULk) of the super-frame structure (10A, 10B), which uplink frame (ULk) comprises a field (UDk) for uplink payload data,
characterized in that
the uplink frame (ULk) comprises a field for time indicating data (T3.k), wherein the transmitting (306) of the uplink response includes transmitting time stamps in the time indicating data field (T3.k), which time stamps indicates the time instance (T³ ₀) of the receiving (302) of the downlink transmission, and the time instance (T³ ₁) of the transmitting (306) of the uplink response, which time instances (T³ ₀, T³ ₁) are determined by means of said clock (31) of the communication node (3.k).
A communication method (300) of claim 1, wherein said uplink frame (ULk) comprises a field (ADLk) for acknowledgements and wherein the transmitting (306) of the uplink response includes transmitting an acknowledgement indicating the receiving (302) of the downlink transmission.
A communication method (300) of claims 1 or 2, wherein the received downlink transmission comprises synchronization data and said method further comprises synchronizing (304) said clock (31) based on the synchronization data, and wherein said synchronizing (304) being performed between the receiving (302) of the downlink transmission, and the transmitting (306) of the uplink response, and said method (300) includes adjusting said time instances (T³ ₀, T³ ₁) in accordance with the synchronization data.
A communication method (300) of any of claims 1 to 3, comprising receiving (312) a further downlink transmission, which further downlink transmission comprises synchronization data, and synchronizing (314) said clock based on the synchronization data of the further downlink transmission.
A communication node (3.k) comprising a communication interface (32), a clock (31) and registering means (33) configured for registering times of reception and transmission and for time stamping transmissions, said communication node (3.k) being configured to perform the communication method (300) of claims 1 or 2.
A communication node (3.k) according to claim 5, further comprising a synchronizer (36) for adjusting the clock (31), said communication node (3.k) being adapted to perform the method (300) of claims 3 or 4.
A communication method (200) for wireless communication using a TDMA super-frame structure (10A, 10B) for scheduling uplink and downlink transmissions, which communication method (200) is performed by a first communication node (2) that comprises a clock (21), which communication method (200) includes transmitting (202) a downlink beacon (11A, 11B) to a plurality of communication nodes (3.1-3.n) and receiving (306) an uplink response from a second node (3.k) of the communication nodes,
characterized in
- registering (203) the time instance (T² ₀) of transmitting the downlink beacon (11A, 11B),

- registering (207) the time instance (T² ₁) of receiving the response (ULk), and
wherein said uplink response comprises time stamps indicating the time instance (T³ ₀) of the reception of the downlink beacon (11A, 11B), and the time instance (T³ ₁) of the transmitting the uplink response by the second node (3.k), and said communication method (200) further includes:
- calculating (208) a clock offset between the first communication node (2) and the second node (3.k),

- calculating (210) the propagation delay between the first communication node (2) and the second node (3.k), which calculating of clock offset (208) and calculating (210) of propagation delay is based on the registered time instances (T² ₀ T² ₁) of the downlink transmission and uplink response and on the time stamps indicating time of reception of the downlink beacon 11A, 11B) and indicating the time of transmission of the response, and

- transmitting (212) a further downlink beacon and including indicating said calculated clock offset and said calculated propagation delay in a field (S3.k) for synchronization data provided in the downlink beacon (11A, 11B).
A communication node (2) comprising a communication interface (22) for wireless communication with other communication nodes, a clock (21) for indicating time, a time register (23) for registering times of receiving and transmitting, a time offset calculator (24) for calculating a time offset of a clock and a propagation delay calculator (25) for calculating a propagation delay between communication nodes, said communication node (2) being configured to perform the communication method (200) of claim 7,
Communication node (2) according to claim 8 further including acknowledging means 27 for acknowledging received transmissions, said communication node (2) being configured to include acknowledgements of received uplink frames in a field in the transmitted downlink beacons.
A communication method (100) for wireless communication, which communication method uses TDMA for communication between a first node (2) and a plurality of second nodes (3.1-3.n), which TDMA communication comprises using a super-frame structure (10) for scheduling uplink data transmissions from the second nodes (3.2-3.n) to the first node (2) and scheduling downlink data transmissions from the first node (2) to the second nodes (3.1-3.n), said method comprising:
- transferring (102) a downlink beacon (11A, 11B) from the first node (2) to the second nodes (3.2-3.n),

- transferring (106) an uplink transmission as a response to the downlink transmission from one second node (3.k), of the second nodes (3.2-3.n), to the first node (2),
characterized in

- calculating (108, 110) a clock offset and a propagation delay between the first node (2) and the second node (3.k) using the time instance (T² ₀) of transmitting (202) the downlink beacon (11A, 11B) by the first node (2), the time instance (T³ ₀) of receiving (302) the downlink beacon (11A, 11B) by the second node (3.k), the time instance (T³ ₁) of transmitting the response (ULk) by the second node (3.k) and the time instance (T² ₁) of receiving the response in the first node (2), and

- synchronizing (104) the first node (2) and the second node (3.k), which synchronization includes transferring (112) a subsequent downlink beacon (11A, 11B) including an indication of said clock offset and said propagation delay in a synchronization field (S3.k) of said further downlink beacon (11A, 11B).
A communication method (100) for wireless communication according to claim 10, further including
- registering (103) the time instance of transfer (T² ₀ T³ ₀) of the first frame (11A, 11B, DLk) by the first node (2) as the time instance of transmitting (T² ₀) and by the second node (3.k) as the time of reception (T³ ₀), and

- registering (105, 107) the time of transfer (T² ₁ T³ ₁) of the response frame (ULk), as the time of transmitting (T³ ₁) by the second node (3.k) and the time of receiving (T² ₁) by the second node (2), wherein said calculating (108, 110) of offset and delay being based on said registered time instances (T² ₀ T³ ₀ T² ₁ T³ ₁).
A communication method (100) for wireless communication according to claim 10 or 11, further including, in the response, a first and a second time stamp in a time indicating field (T3.k) of the second frame (ULk), which first time stamp indicates the time of reception of the first beacon (11A, 11B), and which second time stamp indicates the time of transmission of the response (ULk).
A communication method (100) for wireless communication according to any of claims 10 to 12, further including, in the response, an acknowledgement of the reception of the downlink beacon (11A, 11B) in an acknowledgement field (ADLk) of the second frame (ULk).
A communication system (1) for wireless communication using TDMA (Time Division Multiple Access) between a first node (2) and a plurality of second nodes (3.1-3.n), said communication system (1) comprising a super-frame structure (10) for scheduling uplink data transmissions from the second nodes (3.1-3.n) to the first node (2) and scheduling downlink data transmissions from the first node (2) to the second nodes (3.1-3.n), wherein the super-frame structure (10) comprises a first frame (11) configured for a downlink beacon (11A, 11B), and a set of second frames (UL1-ULn), each second frame (UL1; UL2; ...; ULn) dedicated for uplink transmissions from a respective one (3.1; 3.2; ...; 3n) of the plurality of nodes (3.1-3.n),
characterized in that
each second frame (UL1; UL2; ...; ULn) comprises a field (T3.1; T3.2; ...; T3.n) configured for transmitting time indicating data to the first node (2), or
a fraction of the second frames (UL1-ULn) comprises a field (T3.1; T3.2; ...; T3.m) configured for transmitting time indicating data to the first node (2).
A communication system (1) according to claim 14, said time indicating data comprises a first and a second time stamp, where the first time stamp indicates reception of a downlink frame from the first node (2) and the second time stamp indicates transmission of an uplink frame to the first node (2).
A communication system (1) according to claim 14 or 15, wherein, for each respective node (3.1; 3.2; ...; 3n), each second frame (UL1, UL2, ..., ULn) comprises a field (ADL1; ADL2; ...; ADLn) configured for acknowledgement of reception of downlink data from the first node (2).
A communication system (1) according to any of claims 14 to 16, wherein each second frame (UL1; UL2; ...; ULn) comprises a field (T3.1; T3.2; ...; T3.n) configured for transmitting time indicating data to the first node (2), and for each second node (3.1; 3.2; ...; 3n), said first frame (11) comprises a field (S3.1; S3.2, ...; S3.n) configured for transmitting synchronization data to the second nodes (3.1; 3.2; ...; 3n).
A communication system (1) according to any of claims 14 to 16, wherein a fraction of the second frames (UL1-ULn) comprises a field (T3.1; T3.2; ...; T3.m) configured for transmitting time indicating data to the first node (2), and for each field (T3.1; T3.2; ...; T3.m) of the fraction of the second frames (UL1-ULn), said first frame (11) comprises a field (S3.1; S3.2, ...; S3.m) configured for transmitting synchronization data to the corresponding fraction of the second node (3.1; 3.2; ...; 3m).
A communication system (1) according to claim 17 or claim 18, wherein the synchronization data of said field (S3.1; S3.2, ...; S3.m) (S3.1; S3.2, ...; S3.n) is configured for indicating a propagation delay and a clock offset.
A communication system (1) according to claims 15 and 19, wherein the first node (2) and each second node (3.1; 3.2; ...; 3n) comprises a respective clock (21, 31), and said first node (2) being configured to calculate the clock offset of a second node (3.k) in accordance with the following equation: $Δ = ({T^{3}}_{0} - {T^{2}}_{0} + {T^{3}}_{1} - {T^{2}}_{1}) / 2$
where
Δ is the clock offset of the second node (3.k) in question;

T² ₀ is the time instance when the first node (2) transmits a downlink frame, such as the beacon 11, the time instance being determined by the first node (2);

T² ₁ is the time instance when the first node (2) receives the uplink frame (ULk) from the second node (3.k) according to the clock of the first node (2);

T³ ₀ is the time instance when the second node (3.k) receives the downlink frame, e.g. the beacon 11, according to the clock of the second node (3.k) and as provided to the first node (2) by means of the first time stamp;

T³ ₁ is the time instance when the second node 3.k transmits the uplink frame (ULk) according to the clock of the second node 3.k, and as provided to the first node (2) by means of the second time stamp.
A communication system (1) according to claim 20, wherein the first node (2) is configured to calculate the propagation delay d in accordance with the following equation: $d = {T^{3}}_{0} - {T^{2}}_{0} - Δ$
where
d is the propagation delay between the first node (2) and the second node (3.k) in question, and wherein $Δ = ({T^{3}}_{0} - {T^{2}}_{0} + {T^{3}}_{1} - {T^{2}}_{1}) / 2.$
A communication system (1) according to any of claims 14 to 20, wherein said first frame (11) for each second node (3.1; 3.2; ...; 3n) comprises a field configured for acknowledgement of the reception of a second frame (UL1, UL2, ..., ULn) of a previous super-frame (10).